The invention relates in general to gravity casting of metal components and in particular to gravity casting of vehicle wheels.
Vehicle wheels have a circular wheel disc attached to an annular wheel rim. The wheel disc includes a central wheel hub having a pilot hole and plurality of wheel mounting holes formed therethrough. A plurality of equally circumferentially spaced spokes typically support the wheel hub within the wheel rim. The wheel rim is adapted to support a pneumatic tire.
In the past, vehicle wheels typically have been formed entirely from steel. However, wheels formed from light weight metals, such as aluminum, magnesium and titanium or alloys thereof, are becoming increasingly popular. In addition to weighing less than conventional all-steel wheels, such light weight wheels can be manufactured having a pleasing esthetic shape. Weight savings also can be achieved by attaching a wheel disc formed from a light weight metal alloy to a steel wheel rim.
Light weight wheels are typically formed by forging or casting operations. During a forging operation, a heated billet of the light weight metal alloy is squeezed by very high pressure between successive sets of dies until the final shape of the wheel is formed. During a casting operation, molten metal is poured into a cavity formed in a multi-piece wheel mold. After the metal cools sufficiently to solidify, the mold is opened and a rough wheel casting is removed. The wheel casting is then machined to a final shape. machining can including the outside and inside surfaces of the wheel rim, facing the inboard and outboard wheel disc surfaces and drilling the center pilot hole and the mounting holes through the wheel hub.
Conventional casting operations include numerous processes, such as die casting, low pressure injection casting and gravity casting. All the conventional casting operations typically utilize a wheel mold formed from a number of segments. The wheel mold defines a mold cavity which includes a rim cavity for casting the wheel rim and a disc cavity for casting the wheel disc.
For high volume production of castings, such as vehicle wheels, a highly automated gravity casting process is frequently used. Such automated gravity casting processes typically use a casting machine having a plurality of molds mounted upon a moving structure, such as a rotatable carousel. Each mold is indexed past a refractory furnace containing a pool of molten metal. A charge of molten metal is poured into a gate formed in the mold which communicates with the mold cavity. Gravity causes the metal to flow from the gate into the mold cavity, filling the rim and disc cavities. The mold and the molten metal cool as the casting machine indexes the other molds to the refractory furnace for charging with molten metal. After a sufficient cooling time has elapsed, the mold is opened and the wheel casting removed. The mold is then closed and again indexed to the refractory furnace to be refilled with molten metal.
As the molten metal in the mold cavity cools, it also contracts, or shrinks in volume. Such shrinkage can mar the appearance of the wheel and form voids within the wheel. If a void extends through the wheel, the inflation air of a tire mounted upon the wheel will leak through the void, causing the tire to deflate. Accordingly, cast wheels are carefully checked and wheels which have surface imperfections or which are "leakers" are rejected. In an effort to reduce the number of wheels rejected, wheel molds are usually designed having a relatively large radial spacing between the surfaces of the rim cavity and a relatively large axial spacing between the surfaces of the disc cavity. These spacings produce a wheel casting having an additional volume of metal in the wheel rim and disc. The additional volume of metal allows deeper machining of the wheel casting to remove surface imperfections. The additional volume of metal also results in a wheel rim having a relatively large radial dimension and a wheel disc having a relatively large axial dimension, which reduces the possibility of an internal void extending through the wheel.
To further reduce the number of wheels rejected, a ball riser cavity and a rim riser cavity are typically formed above the center of the disc cavity and at the inboard end of the rim cavity, respectively. These riser cavities receive and store additional molten metal during the pouring operation. As the molten metal within the rim and disc cavities cools and shrinks, gravity draws additional molten metal from the riser cavities into the rim and disc cavities. This additional metal fills any voids that are formed by the shrinkage. The metal remaining in the riser cavities cools to form a center ball riser and an annular rim riser on the casting. Similarly, any metal remaining in the mold gate forms a sprue on the casting. The casting risers and sprue are typically sawed from the casting during the first machining operation.